The invention relates to an anchoring system comprising a vessel, at least two anchors and an anchor line connecting the anchors to the vessel, wherein the anchor line extends from a first anchor, to a pulling device on the vessel, and from the pulling device to the second anchor.
From WO98/45874 it is known to moor a vessel, such as an FPSO, to the sea bed with mooring lines which are secured to the ship""s hull in the central area. By pulling in or paying out the anchor line via securing devices on the side of the vessel, the vessel may be rotated through 180xc2x0 around a vertical axis to be aligned with prevailing wind and current conditions.
From WO-97/47516 an anchoring system for positioning an offshore vessel is known wherein bow and stem anchor lines are connected to respective winches at the bow and the stem of the vessel. The vessel may be rotated by varying the lengths of the anchor lines by means of the winches.
From European patent application EP-A-878389 an anchoring system is known in which bow and stern anchor lines are interconnected and extend in the length direction across the deck of the vessel via a winch for allowing passive semi-weathervaning of the vessel.
The known mooring systems have as a disadvantage that a relative large number of winches need to be applied, which winches have to be of a relatively large size to generate a lifting force directed against the tension in the mooring lines, which may amount to 100 tons or more. Furthermore, the possibilities for station keeping and directional adjustments are relatively limited in the known passive semi-weathervaning system.
It is therefore an object of the present invention to provide a semi-weathervaning anchoring system which utilises a relatively small number of pulling devices of relatively small pulling power. It is a further object of the present invention to provide an anchoring system by which heave and roll motions of the vessel can be significantly reduced, and by which active weathervaning is possible.
Thereto the anchoring system according to the present invention is characterised in that the anchor line is located at a distance from the longitudinal or transverse centreline of the vessel.
By running the anchor line across the vessel at a distance from the longitudinal or transverse centrelines of the vessel through a pulling device, one part of the anchor line can be taken in while the other is paid out to cause a rapid and simple adjustment of the vessel position and/or direction. As one pulling device operates on two anchor lines, the number of pulling devices remains relatively low. Furthermore, as the anchor line parts are tensioned against each other, the pulling device only has to operate against the difference in anchor line tension so that the pulling power of the pulling device can be relatively small.
The vessel of the present invention may be a floating production storage and offloading (FPSO) vessel which may or may not have drilling capabilities (FPDSO) or may be a drilling vessel with a tension leg deck (TLD) such as described in European patent application No. 99200823.5. The anchor lines may be comprised of anchor chains, steel cables, polyester cables, or combinations thereof. The anchor lines may be formed of one piece or may be interconnected at the position of the vessel. The anchor line may comprise two separate sections that each are connected to the pulling device.
The pulling device may comprise a winch, or other equivalent pulling means such as an hydraulic arm or a pinion and rack combination.
The anchors may comprise pile, drag or suction anchors connected to the seabed, and the mooring lines may comprise clump weights or other anchoring equipment for maintaining a stable position of the vessel.
In one embodiment, the anchors are placed on each side of the longitudinal centreline, the pulling device being placed on the forward or rear part of the vessel. By placing the pulling device on the rear part of the vessel, the bow of the vessel can be rotated through about plus or minus 45xc2x0. A 30xc2x0 weathervaining adjustment can already reduce heave and roll motions by as much as 50%.
In a further embodiment both the front and rear parts of the vessel comprise a pulling device such that also lateral displacement and yaw motions of the vessel can be adjusted and/or corrected. Near midship of the vessel, a number of anchor lines that extend in the transverse direction may be fixedly connected or may be connected via additional pulling devices for lateral displacement of the vessel.
Another embodiment comprises an anchor line configuration in which two anchor lines extend in the longitudinal direction of the vessel, on both sides of the longitudinal centreline thereof. Each anchor line comprises a pulling device at the side of the vessel, the anchor lines being guided through fairleads at the bow and at the stem and from thereon continuing towards the anchors.